1. Field of the Invention
The present invention relates to a chrominance signal processing circuit in a color television receiver. More specifically, the present invention relates to an improvement in an automatic color control and an automatic phase control in a color television receiver.
2. Description of the Prior Art
As well known, a composite color television signal comprises a luminance signal and a chrominance signal suppression modulated on a subcarrier in a line scanning period, apart from a horizontal and vertical synchronizing signals in a blanking period and a burst signal at the back porch of the horizontal synchronizing signal. In a typical color television receiver, a luminance signal, horizontal and vertical synchronizing signals, and a chrominance signal plus a burst signal are separated for the purpose of signal processing. For the purpose of processing a chrominance signal, a composite chrominance signal including a chrominance signal suppression modulated on a subcarrier and a burst signal is provided. On the other hand, a burst gate pulse is also provided to sample a burst signal in the composite chrominance signal. A subcarrier is locally generated responsive to the sampled burst signal and the original color signals are demodulated responsive to the chrominance signal and as a function of the locally generated subcarrier.
FIG. 1 shows a block diagram of a typical prior art chrominance signal processing circuit in a color television receiver. Referring to FIG. 1, a composite chrominance signal including a burst signal is applied to a bandpass transformer 1 adjusted to cover the frequency band of the composite chrominance signal. The output of the bandpass transformer 1 is applied to a first bandpass amplifier 2 and further to a second bandpass amplifier 3, wherein the composite chrominance signal is amplified. The output of the second bandpass amplifier 3 is applied to a color gain control, wherein the gain of the bandpass amplifiers 2 and 3 is adjusted. The chrominance signal thus amplified and gain adjusted is applied to an R-Y demodulator 5 and a B-Y demodulator 6, wherein an R-Y color difference signal and a B-Y color difference signal are demodulated as a function of a subcarrier obtained through a line 13 and a subcarrier obtained from a line 11, respectively, both of which have a phase difference of 90.degree. to be described subsequently. More specifically, a subcarrier of the frequency 3.58 MHz is generated by a subcarrier generator 8 and is first applied to a hue or tint control 9, wherein the phase of the subcarrier is manually adjusted by means of a tint control variable resistor 10. The output of the tint control 9 is applied through the line 11 to the B-Y demodulator 6. On the other hand, the output of the tint control 9 is applied to a 90.degree. phase shifter 20, wherein the original subcarrier is phase shifted by 90.degree.. The output of the 90.degree. phase shifter 12 is applied through the line 13 as another subcarrier to the R-Y demodulator 5. The output of the R-Y demodulator 5 and the output of the B-Y demodulator 6 are applied to a G-Y matrix 7, wherein the R-Y color difference signal and the B-Y color difference signal are subjected to an arithmetic operation to provide a G-Y color difference signal.
As well known, various automatic controls such as an automatic color control, an automatic phase control, an automatic color disabling control and the like are employed in a chrominance signal processing circuit. Referring to FIG. 1, an automatic color control will be first described. An automatic color control comprises a burst gate circuit 14 responsive to the burst gate pulse to gate only a burst signal in a composite chrominance signal obtainable from the first bandpass amplifier 2 to provide only a burst signal and a detector 15 for detecting the magnitude of the burst signal obtainable from the burst gate circuit 14 to provide the detected output representing the magnitude of the burst signal through a line 16 to the first bandpass amplifier 2 as a voltage control signal. For the purpose of automatic color control, the first bandpass amplifier 2 is structured in a voltage controlled variable gain amplifier. Therefore, the gain of the first bandpass amplifier 2 is controlled as a function of the output of the detector 15 and thus as a function of the magnitude of the burst signal. Thus, the overall gain of the amplifiers 2 and 3 and the control 4 is automatically controlled as a function of the magnitude of the burst signal. This type of automatic gain control is often referred to as an automatic color control. In FIG. 1, the automatic color control detector 15 is implemented by a synchronous detector operable as a function of the output from the subcarrier generator 8. Thus, the detector 15 is shown responsive to both the output of the burst gate circuit 14 and the output of the subcarrier generator 8.
The output obtainable from the automatic color control detector 15 is not only representative of the magnitude of the burst signal but also of the presence or absence of the burst signal. Therefore, the output obtained through the line 16 from the automatic color control detector 15 is also applied to a color disabling circuit 17 and the output from the color disabling circuit 17 is applied to the second bandpass amplifier 3, so that the second bandpass amplifier 3 is disabled if and when no output is obtained from the automatic color control detector 15 representing the absence of the burst signal and is enabled only when the output representing the presence of the burst signal is obtained from the automatic color control detector 15. This type of automatic color disabling control is often referred to as "color killer".
For the purpose of an automatic hue or phase control, the subcarrier generator 8 is implemented by a voltage controlled variable frequency oscillator and the output of the voltage controlled oscillator 8 is applied through a 90.degree. phase shifter 18 to a phase detector 19, which is also connected to receive the burst signal obtained from the burst gate circuit 14. The phase detector 19 serves to detect the phase difference of the burst signal from the burst gate circuit 14 and of the output of the 90.degree. phase shifter 18. The detected output from the phase detector 19 is applied through an amplifier 20 to the voltage controlled oscillator 8 as a voltage control signal. A closed loop including the voltage controlled oscillator type subcarrier generator 8, the phase detector 19 and the amplifier 20 automatically controls the phase of the output of the subcarrier generator 8 and is often referred to as an automatic phase control.
According to the above described automatic phase control, the output of the first bandpass amplifier 2 is first applied to the burst gate circuit 14, where only the burst signal is sampled or gated and the gated burst signal is used to detect by means of the phase detector 19 the phase of the subcarrier generated by the voltage controlled oscillator type subcarrier generator 8, whereupon the output of the phase detector 19 is applied through the amplifier 20 to the subcarrier generator 8. However, this type of automatic phase control has not taken full advantage of an automatic phase control, inasmuch as no consideration has been paid to a phase drift in the tint control 9, the demodulators 5 and 6, the second bandpass amplifier 3, the color gain control 4 and the like. Conventionally, various countermeasures were taken to eliminate such phase drift as much as possible in the respective circuits. In addition, in order to match the phase of the subcarrier with that of the chrominance signal, a phase compensation circuit 21 was required, because no circuits corresponding to the tint control 9 and the demodulators 5 and 6 were not included in the closed loop for the automatic phase control. Thus, the conventional circuit has an undesirably increased number of portions being adjusted. In addition, the feature of a less temperature drift of the automatic phase control was not effectively utilized.
Similarly, in case of a color gain or color saturation as well, a gain drift in the second bandpass amplifier 3, the color gain control 4, the demodulators 5 and 6, and the like was not automatically corrected, because the above described circuits were not included in the closed loop of the automatic gain control and as a result a gain drift in such circuits caused a variation of the color.